1. Field of the Invention
The present invention relates generally to data transmission in a passive optical network (PON) and, more specifically, to an improvement in the ranging process that is performed prior to initiating (time-division multiplexed) upstream transmissions.
2. Description of the Related Art
Most digital telecommunications networks (i.e., networks that facilitate the communication of data, voice, video, etc., between parties or between a content distribution service and subscribers) typically comprise active components, such as repeaters, relays and other such devices that consume power, in the path between an exchange and a subscriber. In addition to requiring power, active components are subject to failure and performance degradation over time, and may require significant periodic maintenance. The passive optical network (PON) has been developed to overcome some of these deficiencies. The essence of a PON is that nothing but optical fiber and passive components are found in the path between the exchange and subscribers. A single fiber can run from the exchange to a passive splitter located near a group of subscribers, such as a neighborhood or office complex, and individual fibers can run from the splitter to individual subscribers or sub-groups of subscribers.
The International Telecommunications Union (ITU) and the Institute of Electrical and Electronics Engineers (IEEE) are two standards-making bodies currently developing PON standards. The ITU has adopted recommendations of the Full Service Access Networks (FSAN) organization, including G983.x, a specification sometimes referred to as “broadband PON” (BPON), and G984.x, a specification sometimes referred to as “gigabit PON” (GPON). These standards and recommendations are well-known to persons skilled in the art to which the invention relates and are therefore not described in further detail herein (i.e., in this patent specification).
In accordance with these standards and recommendations, a PON comprises an optical line terminator (OLT) at the exchange or central office and a number of optical network units (ONUs), also known as optical network terminals (ONTs), each located at or near the subscriber's premises (e.g., home, office building, etc.), with optical fiber and splitters between the OLT and ONUs. In the downstream direction, i.e., data transmitted from the exchange to a subscriber, the data packets (also referred to as cells) are broadcast from the OLT to all of the ONUs in the network, and an ONU can select the data to receive by matching the address embedded in the data units to a selected address. In the upstream direction, i.e., data transmitted from a subscriber to the exchange, the data units are time-division multiplexed with those transmitted from other subscribers. These systems are sometimes referred to as burst-mode PON technologies because they transmit bursts of data packets at relatively high bit rates.
The amplitude of the upstream signal received at the OLT generally varies from ONU to ONU. The amplitude differences occur for a number of reasons, including the number of splits in the paths and the different distances from the OLT at which the ONUs may be located. Bit errors can occur if the OLT receiver threshold is too high or too low to detect the bits of a packet. To account for differences in amplitudes of signals received from different ONUs, the G984 specifications provide for the OLT to “train” its receiver to each upstream packet, i.e., adjust itself to the amplitude range of that packet, in order to receive the packet data without errors. The specifications provide for inclusion of a preamble preceding the data bits of each packet to use in training the OLT receiver. After an initialization procedure, described below, involving applying a reset signal, the OLT adjusts its receiver's detection threshold upward or downward until the threshold is centered half way between the logic “1” legal and the logic “0” level. When the detection threshold has been successfully centered between the logical levels, the received data will be recovered without duty cycle distortion, which is necessary to the process of recovering the clock signal from the combined clock-data signal.
In accordance with the ITU standards and recommendations, in the upstream direction each ONU is to transmit a data packet only during its assigned timeslot. Nevertheless, because some ONUs may be located at different distances from the OLT than other ONUs, differences in propagation delays may cause data packet transmissions to overlap slightly in time and become garbled with each other at splitters/combiners. A synchronization measure known as “ranging” is employed to prevent such overlap or collisions. Ranging is a method that comprises the OLT calculating a propagation delay between it and each ONU in the PON. The OLT does this by transmitting a ranging window message indicating that ranging is to begin. Any ONU that has not already been ranged responds by transmitting a reply. (Additional steps, not described here for purposes of clarity, are taken to ensure that replies from different ONUs do not collide.) The reply, much like any data packet, includes a delimiter or predetermined bit pattern between the preamble and the data bits that follow the preamble. When the received bits match the expected delimiter, the OLT calculates the time differential between the sending of the ranging grant and the receipt of the ranging response. From the time differential, the OLT can calculate an equalization (EQ) delay for the ONU that will allow the ONU to adjust its upstream transmissions so that they arrive at the OLT precisely within the time slot assigned to that ONU. After the ONU receives its calculated EQ delay from the OLT, ranging is deemed completed for that ONU.
Ranging is generally performed as soon as an ONU is powered on or otherwise brought into the PON, or upon initialization of PON operation, but it can also be performed periodically thereafter to ensure that all ONUs in the PON have been ranged.
As shown in FIG. 1, a prior or conventional OLT has a media access controller (MAC) 10 that controls the majority of OLT functions and thus is analogous to a central processor. The burst-mode transceiver circuitry of the OLT (only the receiver portion 12 of which is shown in FIG. 1 for purposes of clarity) includes an optical module 14, a clock processing device (CPD) 16, and a data de-serializer 18. Optical module 14 receives the optical signal from the PON (i.e., transmitted by an ONU) and detects the waveform transitions that represent the packet preamble and data bits. To detect the data bits without errors, it performs the above-described training process on each packet preamble. That is, it adjusts itself to respond to bit transitions in the amplitude range of the preamble. In PON architectures in which each ONU generates a clock signal that it maintains at the same frequency as the reference clock signal that the OLT generates, CPD 16 is typically a clock phase aligner (CPA) device that merely determines the phase difference between the ONU clock and OLT clock. In other PON architectures, CPD 16 may be a clock-data recovery (CDR) device that recovers a clock signal from the received signal and uses the recovered clock signal to sample the data bits from the received signal. In either case, CPD 16 receives the detected bit stream and outputs a clock signal and an accompanying (serial) data bit stream. In architectures in which CPD 16 is a CPA, it is generally unable to perform its function unless the information it receives is completely undistorted. Regardless of CPD type, all architectures have limits on the amount of duty cycle distortion and jitter they can tolerate. To minimize the amount of distorted preamble information that reaches CPD 16, it is important that the optical module 14 train itself as quickly as possible. De-serializer 18 converts the bit stream to multi-bit words, synchronized with the clock, for use by MAC 12. Note that other portions of the OLT that do not directly relate to the present invention are not shown in FIG. 1 for purposes of clarity.
In a conventional OLT, MAC 12 can assert a reset signal 20 (“RST”) that causes optical module 14 to re-adjust its detection threshold in preparation for receiving a data packet. In other conventional OLT architectures (not shown), the MAC can assert another reset signal that causes the CPD to re-acquire the clock signal. In normal operation, i.e., when the ONUs are transmitting ordinary data packets to OLT 10, MAC 12 can determine with some precision the beginning of a timeslot in which an ONU signal is expected to be received. A problem arises, however, when the packet is not an ordinary data packet, but rather a ranging reply, because MAC 12 cannot determine when a ranging reply will be received. A reply could be received at any time within the ranging window, the length of which is set to a relatively large predetermined worst-case length (e.g., 200 microseconds) to account for ONUs that may be far from the OLT. Due to this uncertainty, it has been suggested that MAC 12 simply assert reset signal 20 immediately after OLT 10 transmits the ranging window message. In some architectures, however, the reset signal 20 causes the optical module 14 to enter a high-gain mode. In the high-gain mode, optical module 14 is unduly sensitive to noise spikes and is thus very susceptible to producing spurious bit outputs. Although not all burst-mode optical receiver architectures necessarily behave in this manner with regard to entering a high-gain mode, essentially all such receivers suffer from the problem that the longer an input (light) signal is absent, the higher the probability that the receiver will produce a spurious output, which will almost always cause the MAC to produce errors. Determining the optimal condition or conditions under which to assert such a reset signal during ranging has been problematic in the art. The present invention addresses this problem and others in the manner described below.